This invention relates generally to surveying of boreholes, and more particularly concerns apparatus and methods to determine azimuth and tilt, employing magnetic means to determine azimuth. This invention also relates to detecting errors in magnetic azimuth, local magnetic anomalies, or nearby magnetic objects.
It is known to use an angular rate sensing gyroscope and an accelerometer in a rotating gimbal to detect both the earth's rotation rate vector and the gravity vector. Each instrument provides an output that is proportional to the vector dot product between the sensor input axis and its associated reference vector, plus certain error terms. Analysis shows that borehole tilt and azimuth may be computed from these sensor outputs, and that the gimbal rotation makes it possible to both measure all required data with only one sensing axis of each kind and to eliminate major bias type error sources from the sensors.
Also, it is known to use magnetometers in borehole surveys; however, no way was known to employ a single magnetometer to determine azimuth, in the unusually advantageous manner as described herein.